Colour calibration

ABSTRACT

A method of color calibration, the method comprising for a user&#39;s display device: a. displaying a set of color regions each region having a sligtly different color; b. receiving an input from a user, the input indicating which color region is considered by the user to represent a unique hue; c. repeating steps a and b for three different unique hues at different intensities; and d. using the user&#39;s selections of unique hues to generate a set of display device color calibration parameters which characterize chromatic properties of the user&#39;s display device.

The present invention relates to colour calibration.

When an image is viewed on a visual display unit (for example a monitor)the appearance of the colours of the image depends upon thecharacteristics of the visual display unit. For this reason, if the sameimage were to be viewed on a different visual display unit theappearance of the colours of the image would be different.

There are some instances in which differences between colour appearanceis not a particular disadvantage, for example when watching television.In the case of a television program the appearance of colours will ingeneral be sufficiently accurate that a viewer's enjoyment of theprogram is not compromised. In the case of a television advert, ingeneral the intention of the advert will be to entice the viewer topurchase the item at a later date, usually at a shop. When buying theproduct the customer (viewer) will be able to see the product itselfbefore deciding whether or not to buy the product. If the colour of theproduct is an important factor in deciding whether or not to buy theproduct, the customer will make the decision based upon the colour ofthe product as seen in the shop; the colour of the product as seen inthe television advert is not used to make the decision.

A significant problem arises when a company advertises products on theWorld Wide Web for direct purchase via the Internet. An image of an itemappearing on the monitor of an end-user will have a colour appearancewhich is different from the actual colour of the item itself. If thecolour of the item is an essential feature of the item this can be aserious commercial problem, since the user may decide to purchase theitem based upon the colour of the viewed image, and will then bedisappointed to find that the colour of the item is not as expected.

It is an object of the present invention to provide colour calibrationwhich overcomes or mitigates the above disadvantage.

According to the invention there is provided a method of colourcalibration, the method comprising for a user's display device:

-   -   a. displaying a set of colour regions each region having a        slightly different colour;    -   b. receiving an input from a user, the input indicating which        colour region is considered by the user to represent a unique        hue;    -   c. repeating steps a and b for three different unique hues at        different intensities; and    -   d. using the user's selections of unique hues to generate a set        of display device colour calibration parameters which        characterise chromatic properties of the user's display device.

The inventor has realised that unique hue settings may be used as aninternal standard to calibrate visual display units. This eliminates theneed for an external standard (e.g. a printed colour chart).

Suitably, steps a and b are repeated thirteen or more times.

Suitably, the three unique hues are selected from the hues red, green,blue and yellow. Suitably, the three unique hues are red, green andblue. Alternatively, the three unique hues may be red, green and yellow.In a further alternative four unique hues may be used, i.e. red, greenblue and yellow.

Suitably, the display device colour calibration defines the orientationof unique hue planes, in a physiologically defined colour space, of thedisplay device.

Suitably, the display device colour calibration defines offsets of thedisplay device.

Suitably, the display device colour calibration defines gain of thedisplay device.

Suitably, the display device colour calibration defines a valueindicative of the non-linearity of the display device.

Suitably, the display device colour calibration is stored as a file.

Suitably, the stored display device colour calibration file isassociated with an image found to have substantially veridical colourwhen viewed on the user's display device.

Suitably, the stored display device colour calibration file is retainedfor future use.

Suitably, the method further comprises receiving an image together witha set of image colour calibration parameters, the image colourcalibration parameters characterising chromatic properties of anoriginator's display device upon which the image was formed to havesubstantially veridical colour, using the image colour calibrationparameters and the display device colour calibration parameters togenerate a transformation, the transformation indicating adjustments tobe made to the image in order that the image be displayed on the user'sdisplay device with substantially veridical colour, applying thetransformation to the image and displaying the resulting image on theuser's display device.

Suitably, the transformation determines how pixel values of the imageshould be transformed to account for chromatic properties of the user'sdisplay device.

Suitably, the transformation maps unique hue planes generated on theoriginator's display device to unique hue planes generated on the user'sdisplay device.

Suitably, the image colour calibration parameters comprise a storeddisplay device colour calibration file for the originator's displaydevice.

Suitably, the colour regions are patches arranged as an annulus.

Suitably, the display device is a visual display unit.

Suitably, the display device is a printer.

Suitably, steps a and b are repeated for a fourth unique hue.

The invention also provides a colour calibration file which defines theorientation of unique hue planes of a display device.

Suitably, the colour calibration file defines offsets of the displaydevice.

Suitably, the colour calibration file defines gain of the displaydevice.

Suitably, the colour calibration file defines a value indicative of thenon-linearity of the display device.

The invention also provides a colour calibration apparatus for a user'sdisplay device comprising:

-   -   a. means for displaying a set of colour regions each region        having a slightly different colour;    -   b. means for receiving an input from a user, the input        indicating which colour region is considered by the user to        represent a unique hue;    -   c. means for repeating steps a and b for three different unique        hues at different intensities; and    -   d. means for using the user's selections of unique hues to        generate a set of display device colour calibration parameters        which characterise chromatic properties of the user's display        device.

A specific embodiment of the invention will now be described by way ofexample only with reference to the following accompanying drawings, inwhich:

FIG. 1 is a schematic representation of a calibration test according tothe invention, as seen on a visual display unit of an originator;

FIG. 2 is a schematic representation of the outcome of a typicalcalibration test performed by the originator;

FIG. 3 is a schematic representation of the calibration test as seen ona visual display unit of a user;

FIG. 4 is a schematic representation of the outcome of a typicalcalibration test performed by the user; and

FIG. 5 is a flow chart of the calculation and the calibration steps.

An originator C creates an image on his/her visual display unit A. Theoriginator C may for example be an employee of a company which sellsclothes on the World Wide Web, and the image may be for example an itemof clothing. The originator C during creation of the image checks thatthe colour of the image is correct, for example by checking that thecolour of the image of the item of clothing corresponds to the colour ofthe item of clothing itself.

Once the originator C is satisfied with the colour appearance of theimage, he/she performs a colour calibration for the visual display unitA.

The colour calibration is based upon four hues: red, green, yellow andblue (these are known as the unique hues). It is known that the uniquehues are approximately constant across cultures, gender, age, race etc.That is, given the same visual display unit, most human observers withnormal colour vision make very similar and accurate judgements about theunique hues. In other words, when asked to select a specific unique huefor a visual display unit, human observers pick the same combination ofintensities of the red, green, and blue phosphor of the visual displayunit. This constancy is exploited by the invention, which uses theconstancy as a standard when calibrating different visual displaysystems.

Referring to FIG. 1, an annulus of similarly coloured test patches B isdisplayed on the visual display unit A of the originator C. Theoriginator C selects the patch most similar to the unique hue, forexample denoted by G in FIG. 2. The originator C will use an inputdevice, for example a computer mouse, to make his/her selection. Theselection is performed for three unique hues; hence the annulus B willalways consist of bluish, reddish, or greenish colours and the task ofthe originator C is always to select the patch that is closest to theunique blue, red, or green, respectively. This is repeated five timesfor different intensities of each of unique blue, red, and green.

It is important to note that the originator C is not being asked tocompare the coloured test patches with a set of printed referencecolours, or some other set of reference colours provided to theoriginator. Instead, the originator C is simply asked to select forexample the red colour patch which is ‘neither yellow nor blue’.Similarly, unique green is the green colour patch which is ‘neitheryellow not blue’, whereas unique blue is the blue colour patch which is‘neither red nor green’. The calibration therefore takes advantage ofthe ability of human observers with normal colour vision to make verysimilar and accurate judgements about the unique hues.

As previously mentioned, the calibration procedure requires a selectionto be made by the originator C of a colour patch for the unique huesred, green and blue (unique yellow is not used in this example). The RGBvalues (the intensities of the red, green, and blue phosphor) which gaverise to each of the unique colours, as selected by the originator, areaccessed and are used to generate a calibration file H for theoriginator's visual display unit. This is shown schematically in FIG. 2.The calibration file H contains all the relevant information about theoriginator's visual display unit A. The calibration file H is associatedwith the image, so that when the image is sent to a user F thecalibration file H is also sent to the user F.

The image that was created by originator A is received, together withthe originator's calibration file H, by user F. The image and filetransmission is typically via the Internet, for example by downloading,ftp or email. In general, the user F views the image on a visual displayunit D which has different characteristics to the visual display unit Aused by the originator.

Upon receiving the image, the user F is asked whether he/she would liketo perform a colour calibration so that/the colour of the viewed imageis as intended by the originator. Upon request by the user, thecalibration procedure is performed by user F on the visual display unitD, as shown in FIG. 3. An annulus of similarly coloured test patches Eis displayed and the user F selects the patch most similar to the uniquehue, for example denoted by I in FIG. 4. This selection by the user F isagain made by an input device, for example a computer mouse. The RGBvalues of the selected patch are stored. The process is repeated togenerate a calibration file J for the unique hues red, green and blue.

As shown in FIG. 5, both calibration files H and J are accessed, and atransformation K is estimated based upon the RGB values corresponding tothe unique hues on the user's display unit D and the originator'sdisplay unit A. This transformation K is applied to the RGB values ofthe user's visual display unit.

Application of the transformation K to the RGB values of the visualdisplay unit D of the user F will result in the image created by theoriginator C having an appearance identical to the image viewed by theuser F. Hence a true device-independent calibration has been achieved.

The calibration will now be described in more detail:

For each of the three unique hues, a plane in a physiologically-definedthree-dimensional space is determined which fits to the observer'sunique hue settings. The dimensions of the three-dimensional space arethe light intensities that correspond to the RGB values. The RGB valuesare device-specific, and typically range from 0 to 255 (8 bit). Thuseach unique hue setting (e.g. unique red) is described by an RGB vector(for example R=230, G=150, B=200).

The relationship between the light output (intensity) of each phosphorand the internal pixel value (R,G,B) is non-linear and is described by atransfer function of the following form:Intensity I∝(k·Pixel_value+l)^(γ)  (Eq. 1)where k and l are the system gain and offset values; the gamma valuereflects the non-linearity. The gain (k) and the offset (l) are to someextent under the control of the user in the form of contrast andbrightness knobs. In Cathode-Ray-Tube based monitors, the non-linearity(γ) reflects the non-linear relationship between the voltage applied tothe CRT electron-gun and the visible radiant energy (light output). Formost CRT-based monitors the gamma value is between 1.8 and 2.2. Otherdisplay devices may have entirely different gamma values or, moregenerally, different transfer functions.

The unique hues may be described as planes in non-linear RGB space,where the non-linear RGB space is light output rather than internalpixel values. Given the transfer function (Eq. 1), the planes of theunique hues in non-linear RGB space are defined by a set of equations ofthe form:

Unique Red Plane({tilde over (R)}): a_({tilde over (R)})(k _(R) R+l _(R))^(γ) +b_({tilde over (R)})(k _(G) G+l _(G))^(γ) +c _({tilde over (R)})(k _(B)B+l _(B))^(γ) +d _({tilde over (R)})=0

Unique Green Plane({tilde over (G)}): a_({tilde over (G)})(k _(R) R+l _(R))^(γ) +b_({tilde over (G)})(k _(G) G+l _(G)) ^(γ) +c _({tilde over (G)})(k _(B)B+l _(B))^(γB) +d _({tilde over (G)})=0

Unique Blue Plane({tilde over (B)}): a_({tilde over (B)})(k _(R) R+l _(R))^(γ) +b_({tilde over (B)})(k _(G) G+l _(G)) ^(γ) +c _({tilde over (B)})(k _(B)B+l _(B))^(γ) +d _({tilde over (B)})=0

Since each of the unique hue planes (i=<{tilde over (R)},{tilde over(G)},{tilde over (B)}>) contains the origin (black), the constants(d_(i)) are set to zero. One parameter can be eliminated by dividing theequations by c_(i) which further simplifies the equations to thefollowing:

$\text{Unique~~Red~~Plane~~}( \overset{\sim}{R} )\text{:}$${{\frac{a_{\overset{\sim}{R}}}{c_{\overset{\sim}{R}}}( {{k_{R}R} + l_{R}} )^{\gamma}} + {\frac{b_{\overset{\sim}{R}}}{c_{\overset{\sim}{R}}}( {{k_{G}G} + l_{G}} )^{\gamma}} + ( {{k_{B}B} + l_{B}} )^{\gamma}} = 0$$\text{Unique~~Green~~Plane~~}( \overset{\sim}{G} )\text{:}$${{\frac{a_{\overset{\sim}{G}}}{c_{\overset{\sim}{G}}}( {{k_{R}R} + l_{R}} )^{\gamma}} + {\frac{b_{\overset{\sim}{G}}}{c_{\overset{\sim}{G}}}( {{k_{G}G} + l_{G}} )^{\gamma}} + ( {{k_{B}B} + l_{B}} )^{\gamma}} = 0$$\text{Unique~~Blue~~Plane~~}( \overset{\sim}{B} )\text{:}$${{\frac{a_{\overset{\sim}{B}}}{c_{\overset{\sim}{B}}}( {{k_{R}R} + l_{R}} )^{\gamma}} + {\frac{b_{\overset{\sim}{B}}}{c_{\overset{\sim}{B}}}( {{k_{G}G} + l_{G}} )^{\gamma}} + ( {{k_{B}B} + l_{B}} )^{\gamma}} = 0$

To further simplify the equations we re-define a_(i) as a_(i)/c_(i), andb_(i) as b_(i)/c_(i) and replace the non-linear transfer function (Eq 1)with the intensity I:

$\begin{matrix}{{\begin{bmatrix}a_{\overset{\sim}{R}} & b_{\overset{\sim}{R}} & 1 \\a_{\overset{\sim}{G}} & b_{\overset{\sim}{G}} & 1 \\b_{\overset{\sim}{G}} & b_{\overset{\sim}{B}} & 1\end{bmatrix}\begin{bmatrix}\begin{matrix}I_{R} \\I_{G}\end{matrix} \\I_{B}\end{bmatrix}} = \begin{bmatrix}\begin{matrix}0 \\0\end{matrix} \\0\end{bmatrix}} & ( {{Eq}.\mspace{14mu} 2} )\end{matrix}$Therefore, for each of the three unique hue planes, we need to estimatethe vector [a_(i), b_(i), l], which in total requires six parameters.The vector [a_(i), b_(i), l] is normal to the respective unique hueplane. The remaining seven unknown parameters, namely the three gainvalues (k_(R), k_(G), k_(B)), the three offsets (l_(R), l_(G), l_(B)),and the gamma value (γ) are the same in all three equations resulting ina total of 13 parameters.

To estimate the 13 parameters, the observer makes five settings for eachof the unique hues resulting in a set of 15 equations. A standardminimisation procedure is used to estimate the 13 parameters from the 15equations. The standard minimisation procedure may be for example theNelder-Mead simplex method (which is used by the proprietary softwareMATLAB), the gradient descent method or Newton's method. The 13estimated parameters completely characterise the chromatic properties ofthe display device of the originator. The three normal vectors [a_(i),b_(i), l] determine the orientation of the unique hue plane's of displaydevice A; the offsets (l_(R), l_(G), l_(B)), the gain values (k_(R),k_(G), k_(B)), and the gamma value (γ) characterise the non-linearity ofthe display device A.

The 13 parameters are stored in the originator's calibration file H (seeFIG. 2). The originator's calibration file H contains all the relevantinformation about the visual display unit A and is sent together withthe image to user F.

The user F carries out the same calibration as described above forhis/her display device D. Using the technique described above, a set ofequations analogous to Eq. 2 are obtained for display device D:

$\begin{matrix}{{\begin{bmatrix}a_{\overset{\sim}{R}}^{\prime} & b_{\overset{\sim}{R}}^{\prime} & 1 \\a_{\overset{\sim}{G}}^{\prime} & b_{\overset{\sim}{G}}^{\prime} & 1 \\b_{\overset{\sim}{G}}^{\prime} & b_{\overset{\sim}{B}}^{\prime} & 1\end{bmatrix}\begin{bmatrix}\begin{matrix}I_{R}^{\prime} \\I_{G}^{\prime}\end{matrix} \\I_{B}^{\prime}\end{bmatrix}} = \begin{bmatrix}\begin{matrix}0 \\0\end{matrix} \\0\end{bmatrix}} & ( {{Eq}.\mspace{14mu} 3} )\end{matrix}$

The 13 parameters referred to above in relation to display device A areestimated for display device D and are stored in the user's calibrationfile J. The user's calibration file J is the profile for display unit Dof user F and contains all the relevant information required for displayunit D.

The calibration procedure takes both calibration files H and J, andfinds the transformation that maps the unique hue planes generated ondisplay unit A into the unique hue planes generated on display unit D(this is shown schematically in FIG. 5). From Eq. 2 and Eq. 3 it followsthat:

${\begin{bmatrix}a_{\overset{\sim}{R}} & b_{\overset{\sim}{R}} & 1 \\a_{\overset{\sim}{G}} & b_{\overset{\sim}{G}} & 1 \\b_{\overset{\sim}{G}} & b_{\overset{\sim}{B}} & 1\end{bmatrix}\begin{bmatrix}\begin{matrix}I_{R} \\I_{G}\end{matrix} \\I_{B}\end{bmatrix}} = {\begin{bmatrix}a_{\overset{\sim}{R}}^{\prime} & b_{\overset{\sim}{R}}^{\prime} & 1 \\a_{\overset{\sim}{G}}^{\prime} & b_{\overset{\sim}{G}}^{\prime} & 1 \\b_{\overset{\sim}{G}}^{\prime} & b_{\overset{\sim}{B}}^{\prime} & 1\end{bmatrix}\begin{bmatrix}\begin{matrix}I_{R}^{\prime} \\I_{G}^{\prime}\end{matrix} \\I_{B}^{\prime}\end{bmatrix}}$

In the following the coefficient matrices consisting of the normalvectors for display unit A and D are denoted as N_(A) and N_(D)respectively. The intensity vectors for display unit A and D are denotedas I_(A)=[I_(R), I_(G), I_(B)] and I_(D)=[I_(R′), I_(G′), I_(B′)],respectively. SinceN_(A)I_(A)=N_(D)I_(D),it follows thatI_(D)=N_(A) ⁻¹N_(D)I_(A),where N_(A) ⁻¹ is the inverse of N_(A). Defining N_(AD)=N_(A) ⁻¹N_(D)results in the following:I_(D)=N_(AD)I_(A)  (Eq. 4)N_(AD) is a matrix that maps the intensities of visual display unit Ainto the intensities of visual display unit D. Equation 4 determines howthe intensities of visual display unit D need to be transformed so thatan image on display unit A will appear identical to the same imageviewed on display unit D.

Since the calibration procedure has no access to the intensities (lightoutputs) of each colour channel but only to the internal pixel values(R,G,B) equation 4 needs to be solved for the pixel values (R,G,B).Since the transfer function relating the RGB pixel values to theintensities has been estimated (from Eq. 1 and 2) we can substitute theintensities with the actual pixel values (R,G,B). From Equation 1 and 4it follows:

$( \begin{bmatrix}{{k_{R^{\prime}}R^{\prime}} + l_{R^{\prime}}} \\{{k_{G^{\prime}}G^{\prime}} + l_{G^{\prime}}} \\{{k_{B^{\prime}}B^{\prime}} + l_{B^{\prime}}}\end{bmatrix} )^{\gamma^{\prime}} = {\lbrack N_{AD} \rbrack( \begin{bmatrix}{{k_{R}R} + l_{R}} \\{{k_{G}G} + l_{G}} \\{{k_{B}B} + l_{B}}\end{bmatrix} )^{\gamma}}$where the non-linearity γ is applied to each of the three vector entriesseparately. Rewriting the left-hand side yields

$( {\begin{bmatrix}{k_{R^{\prime}}R^{\prime}} \\{k_{G^{\prime}}G^{\prime}} \\{k_{B^{\prime}}B^{\prime}}\end{bmatrix} + \begin{bmatrix}l_{R^{\prime}} \\l_{G^{\prime}} \\l_{B^{\prime}}\end{bmatrix}} )^{\gamma^{\prime}} = {\lbrack N_{AD} \rbrack( \begin{bmatrix}{{k_{R}R} + l_{R}} \\{{k_{G}G} + l_{G}} \\{{k_{B}B} + l_{B}}\end{bmatrix} )^{\gamma}}$By taking both sides of the equation to the power of (1/γ′) results in

$( {\begin{bmatrix}{k_{R^{\prime}}R^{\prime}} \\{k_{G^{\prime}}G^{\prime}} \\{k_{B^{\prime}}B^{\prime}}\end{bmatrix} + \begin{bmatrix}l_{R^{\prime}} \\l_{G^{\prime}} \\l_{B^{\prime}}\end{bmatrix}} ) = \{ {\lbrack N_{AD} \rbrack( \begin{bmatrix}{{k_{R}R} + l_{R}} \\{{k_{G}G} + l_{G}} \\{{k_{B}B} + l_{B}}\end{bmatrix} )^{\gamma}} \}^{1/\gamma^{\prime}}$and by subtracting the offset vector we obtain:

$\begin{bmatrix}{k_{R^{\prime}}R^{\prime}} \\{k_{G^{\prime}}G^{\prime}} \\{k_{B^{\prime}}B^{\prime}}\end{bmatrix} = {\{ {\lbrack N_{AD} \rbrack( \begin{bmatrix}{{k_{R}R} + l_{R}} \\{{k_{G}G} + l_{G}} \\{{k_{B}B} + l_{B}}\end{bmatrix} )^{\gamma}} \}^{1/\gamma^{\prime}} - \begin{bmatrix}l_{R^{\prime}} \\l_{G^{\prime}} \\l_{B^{\prime}}\end{bmatrix}}$We can write the gain coefficients as a diagonal matrix:

${\begin{bmatrix}k_{R^{\prime}} & 0 & 0 \\0 & k_{G^{\prime}} & 0 \\0 & 0 & k_{B^{\prime}}\end{bmatrix}\begin{bmatrix}\begin{matrix}R^{\prime} \\G^{\prime}\end{matrix} \\B^{\prime}\end{bmatrix}} = ( {\{ {\lbrack N_{AD} \rbrack( \begin{bmatrix}{{k_{R}R} + l_{R}} \\{{k_{G}G} + l_{G}} \\{{k_{B}B} + l_{B}}\end{bmatrix} )^{\gamma}} \}^{1/\gamma^{\prime}} - \begin{bmatrix}l_{R^{\prime}} \\l_{G^{\prime}} \\l_{B^{\prime}}\end{bmatrix}} )$Multiplying the equation with the inverse of the gain matrix yields:

$\begin{matrix}{\begin{bmatrix}\begin{matrix}R^{\prime} \\G^{\prime}\end{matrix} \\B^{\prime}\end{bmatrix} = {\begin{bmatrix}{1/k_{R^{\prime}}} & 0 & 0 \\0 & {1/k_{G^{\prime}}} & 0 \\0 & 0 & {1/k_{B^{\prime}}}\end{bmatrix}( {\{ {\lbrack N_{AD} \rbrack( \begin{bmatrix}{{k_{R}R} + l_{R}} \\{{k_{G}G} + l_{G}} \\{{k_{B}B} + l_{B}}\end{bmatrix} )^{\gamma}} \}^{1/\gamma^{\prime}} - \mspace{644mu}\lbrack \begin{matrix}l_{R^{\prime}} \\l_{G^{\prime}} \\l_{B^{\prime}}\end{matrix} \rbrack} )}} & ( {{Eq}.\mspace{14mu} 5} )\end{matrix}$

The left-hand side of Equation 5 are the unknown RGB values of thedisplay unit D of the user F. The right-hand side of the equationcontains only known parameters. This transformation is called K in FIG.5 and is based on the two calibration files H and J.

The transformation K (defined in Equation 5) determines how the pixelvalues (R, G, B) of an image generated on display unit A need to betransformed to account for the different chromatic properties of the twodisplay units. After the transformation K has been applied to the RGBvalues of the image, the image created by originator C on visual displayunit A will look identical to the image viewed by the user F on visualdisplay unit D. Hence the calibration procedure has achieved a truedevice-independent calibration.

The calibration procedure need only be carried out once for any givenvisual display device, the resulting calibration file being stored forfuture use (the characteristics of the display device will not changesignificantly).

The invention may be used for any visual display device, and is notrestricted to traditional cathode ray tube (CRT) displays. The inventionmay for example be used for liquid crystal displays (LCDs).

The calibration procedure may be used for image or video applicationsinvolving the world-wide web. It may also be used for image transferusing ftp or email when a veridical image representation is required.

The calibration procedure can run on any platform (e.g. Windows, UNIX ora Macintosh operating system) since it can be implemented using JAVA orCGI.

The calibration procedure does not require a visual display device tohave a specific colour resolution. It is usually the case that 255different pixel values are available for each of the three colourchannels of a visual display device. However, the calibration procedurecan be used for devices with a higher or a lower resolution.

Any suitable input device may be used by a user to select the uniquehues, for example a keyboard, a mouse, a joystick, touch screen, etc.

The calibration procedure does not depend on the originator and the userhaving similar display devices. For example, the originator may use aCRT based monitor, whereas the user may view the image on a LCD panel.The transformation K is general and does not rely on similar spectralpower distributions for both display devices. Since the calibrationprocedure is device independent, it may be applied to future displaydevices having colour channels with different chromatic properties.

The derivation of the transformation K which provides mapping of coloursbetween display devices is presented as an example. It will beappreciated that other algorithms could be used to find the mapping. Anymapping should be based on the unique hues settings and should map theRGB space from device A into the RGB space of device D.

The described embodiment of the invention includes the assumption thatthe non-linearity (gamma values in equation 1) is the same in the colourchannels: (e.g. red, green, blue phosphors. If the gamma values differin the n (usually 3) chromatic channels, (n−1) more parameters need tobe estimated.

The transfer function specified in equation 1 may be of a differentnature for different display systems. The transfer function specified inequation 1 describes well the light output for currently availableCRT-based display devices. It will be appreciated that other transferfunctions may be used as appropriate for other types of display device,and that the calibration procedure will work in the same manner. Moreparameters may be required to estimate the transfer function.

Although the described example uses the three unique hues red, green andblue, it will be appreciated that the invention could be implementedusing the unique hues red, green and yellow. It is noted that uniqueyellow and unique blue lie on a plane in colour space and are linearlydependent. Thus, all four unique hues may be used to obtain a moreprecise estimate of the unique yellow-blue plane.

The e-calibration tool can be adapted to be used for device-independentprinter calibration. Only a minor modification of the calibrationprocess is required. The following steps are involved:

-   -   (i) The originator generates the image and the calibration        file (H) for display unit A    -   (ii) The user receives the image and performs a unique hue        selection task using a printed version of the annuli of coloured        patches (FIG. 1).    -   (iii) A calibration file is generated for user F; this        calibration file is based on the perceptual judgements of the        printed stimuli, rather than stimuli viewed on a monitor. This        Calibration File (J) contains the profile of the printer and all        the relevant information about the chromatic properties of the        printer.    -   (iv) The transformation K is again calculated from the two        device profiles H and J.    -   (v) The transformation K is applied to the pixel values in each        of the three colour channels. After the correction with the        transformation K the printed version of the image should look        identical to the image viewed on the visual display unit.

1. A method of colour calibration, the method comprising for a user'sdisplay device: a. displaying a set of test colour regions to a user,each region having a slightly different colour; b. receiving an inputfrom the user, the input indicating which colour region among the set oftest colour regions is considered by the user to represent a unique hue,the unique hue being one of unique yellow, unique red, unique green, andunique blue, unique yellow being a yellow which is neither red or green,unique red being a red which is neither yellow or blue, unique greenbeing a green which is neither yellow or blue, and unique blue being ablue which is neither red or green; c. repeating steps a and b for atleast three different unique hues at different intensities; and d. usingthe user's selections of the at least three different unique hues togenerate a set of display device colour calibration parameters whichcharacterise chromatic properties of the user's display device.
 2. Amethod according to claim 1, wherein steps a and b are repeated thirteenor more times.
 3. A method according to claim 1, wherein the at leastthree different unique hues are unique red, unique green and uniqueblue.
 4. A method according to claim 1, wherein the at least threedifferent unique hues are unique red, unique green and unique yellow. 5.A method according to claim 1, wherein the user's selections of fourunique hues are used, the four unique hues being unique red, uniquegreen, unique blue and unique yellow.
 6. A method according to claim 1,wherein the display device colour calibration defines the orientation ofunique hue planes, in a physiologically defined colour space, of thedisplay device.
 7. A method according to claim 1, wherein the displaydevice colour calibration defines offsets of the display device.
 8. Amethod according to claim 1, wherein the display device colourcalibration defines gain of the display device.
 9. A method according toclaim 1, wherein the display device colour calibration defines a valueindicative of the non-linearity of the display device.
 10. A methodaccording to claim 1, wherein the display device colour calibration isstored in a file.
 11. A method according to claim 10, wherein the storeddisplay device colour calibration file is associated with an image foundto have substantially veridical colour when viewed on the user's displaydevice.
 12. A method according to claim 10, wherein the stored displaydevice colour calibration file is retained for future use.
 13. A methodaccording to claim 1, wherein the method further comprises receiving animage together with a set of image colour calibration parameters, theimage colour calibration parameters characterising chromatic propertiesof an originator's display device upon which the image was found to havesubstantially veridical colour, using the image colour calibrationparameters and the display device colour calibration parameters togenerate a transformation, the transformation indicating adjustments tobe made to the image in order that the image be displayed on the user'sdisplay device with substantially veridical colour, applying thetransformation to the image and displaying the resulting image on theuser's display device.
 14. A method according to claim 13, wherein thetransformation determines how pixel values of the image should betransformed to account for chromatic properties of the user's displaydevice.
 15. A method according to claim 13, wherein the display devicecolour calibration defines the orientation of unique hue planes, in aphysiologically defined colour space, of the display device; and whereinthe transformation maps unique hue planes generated on the originator'sdisplay device to unique hue planes generated on the user's displaydevice.
 16. A method according to claim 13, wherein the stored displaydevice colour calibration file is associated with an image found to havesubstantially veridical colour when viewed on the user's display device;and wherein the image colour calibration parameters comprise a storeddisplay device colour calibration file for the originator's displaydevice.
 17. A method according to claim 1, wherein the colour regionsare patches arranged as an annulus.
 18. A method according to claim 1,wherein the display device is a visual display unit.
 19. A methodaccording to claim 1, wherein the display device is a printer.
 20. Amethod according to claim 1, wherein steps a and b are repeated for afourth unique hue.
 21. A colour calibration apparatus for a user'sdisplay device comprising: a. means for displaying a set of test colourregions to a user, each region having a slightly different colour; b.means for receiving an input from the user, the input indicating whichcolour region among the set of test colour regions is considered by theuser to represent a unique hue, the unique hue being one of uniqueyellow, unique red, unique green, and unique blue, unique yellow being ayellow which is neither red or green, unique red being a red which isneither yellow or blue, unique green being a green which is neitheryellow or blue, and unique blue being a blue which is neither red orgreen; c. means for repeating steps a and b for at least three differentunique hues at different intensities; and d. means for using the user'sselections of the at least three different unique hues to generate a setof display device colour calibration parameters which characterisechromatic properties of the user's display device.
 22. A methodaccording to claim 21, wherein the at least three different unique huesare unique red, unique green and unique blue.
 23. A method according toclaim 21, wherein the at least three different unique hues are uniquered, unique green and unique yellow.
 24. A method according to claim 11,wherein the stored display device colour calibration file is retainedfor future use.
 25. A method according to claim 14, wherein the displaydevice colour calibration defines the orientation of unique hue planes,in a physiologically defined colour space, of the display device; andwherein the transformation maps unique hue planes generated on theoriginator's display device to unique hue planes generated on the user'sdisplay device.